The present invention relates to a process for the production of a thermosetting, preferably decorative laminate.
Products with a surface layer of thermosetting resin are frequently used today. They are used mostly where the demands for abrasion resistance are high but also where resistance against different chemicals and humidity is requested. Flooring materials, skirtings, work tops, table tops, cupboards, wall panels and facades can be mentioned as examples on such products. The thermosetting laminate often consists of a number of base sheets with a closest to the surface placed decor sheet of paper and on top of the decor sheet usually one or more protective so-called overlay sheets of xcex1-cellulose.
Thermosetting laminates are produced by laminating together at an increased pressure paper sheets or paper webs impregnated with a thermosetting resin. In respect of paper sheets this is made in a static press and in respect of paper webs in continuous presses. Continuous laminate presses have been known for a long time and they are manufactured for instance by the companies Kvaerner and Hymmen. The presses are furnished with two endless belts. The laminate is pressed between these belts.
For a long time there has been a great need of being able to produce a decorative thermosetting laminate in a rational a cost-effective way without decreasing the quality demands. One such method would be to produce two or more laminates at the same time in continuous presses. However, this has been considered to be impossible.
According to the present invention it has been possible to meet the above need whereby the cost efficiency at the production of thermosetting laminates has been considerably improved. Thus, the invention relates to a process for the production of a thermosetting preferably decorative laminate. The laminate is made of a number of paper sheets preferably in the form of continuous webs impregnated with a thermosetting resin and then dried. The dry paper webs are fed between the press belts of a continuous double belt press and laminated together in the press at an increased pressure and an increased temperature. The invention is characterised in that the continuous paper webs are divided into at least two groups each consisting of at least two sheets of which at least one is impregnated with a thermosetting resin, the sheets consisting of
a) one or more decorative paper webs preferably impregnated with a thermosetting resin and/or
b) one or more overlay paper webs of xcex1-cellulose preferably impregnated with a thermosetting resin and/or
c) one or more bottom colour paper webs preferably impregnated with a thermosetting resin and/or
d) one or more base paper webs preferably impregnated with a thermosetting resin and one or more separation sheets which are placed between the groups of paper webs. The separation sheets are used for separating the groups of paper webs from each other after a finished pressing whereby at least two thermosetting laminates are obtained.
According to one embodiment of the invention the separation sheet for each group consists of an unimpregnated paper web, a paper web impregnated with a thermosetting resin on one side or a paper web coated with a thermosetting resin on one side. The unimpregnated or uncoated side is turned away from the laminate with the possibly coated or impregnated side turned inwards towards the laminate.
Alternatively the separation sheet for each group can consist of an impregnated paper web, an unimpregnated paper web, a paper web impregnated with a thermosetting resin on a first side or a paper web coated with a thermosetting resin on a first side while the other side of the web is coated or impregnated with a passive material. The first side is turned towards the respective group. The passive material then suitably consists of alginate, a metal such as aluminium or plastics such as electron cured acrylate, polyphenyl oxide, polyimide or polyvinylidene fluoride.
According to a second embodiment of the invention the separation consists of one or more separation webs.
A first and a second group of paper webs each comprising a decor paper web are used according to one embodiment of the invention. The decor paper web of the first group is directed with the decor towards the upper press belt with possible intermediate overlay paper webs and possible further underlying decor paper webs, bottom colour paper webs and base paper webs. The decor paper web of the second group is directed with the decor towards the lower press belt with possible intermediate overlay paper webs and possible further decor paper webs placed above, bottom colour paper webs and base paper webs. One or more separation webs are placed between the first group and the second group. Further groups of paper webs with intermediate separation webs can be fed between the press belts together with the first and the second group.
A first structure web is suitably placed between the upper press belt and the first group of paper webs. A second structure web is suitably placed between the lower press belt and the lower group of paper webs. The surface structure of the two structure webs is directed towards the respective paper webs, whereby the two laminates produced at the pressing are provided with a surface structure from the structure webs.
Alternatively the press belts of the continuous press can be furnished with a surface structure. Then the structure webs are eliminated.
A surface structure is above all used for increasing the realism of the decor of the decor paper web. The decor paper web can be provided with a decor or pattern as desired. Usual patterns tend to imitate different kinds of wood or mineral such as marble or granite. The decor can also be a wholly fancy pattern. The decor of the structure web gives a negative image thereof in the laminate at the pressing. The structure suitable reproduces characteristic features of the pattern the decor of the laminate represents. The structure can here be made rough to simulate for example a rough-hewn stone or be glossy with small pits and micro cracks placed at random to simulate polished marble. In order to simulate the surface structure of wood the laminate surface is furnished with thin elongated recesses to look like pores. These recesses must be turned in the growth direction of the simulated tree which is shown by the pattern of the graining to give a realistic result. Therefore, the graining and the simulated pores are often placed in the longitudinal direction of the laminate.
According to an alternative embodiment of the invention a first and a second group of paper webs each includes a decor paper web. The decor paper web of the first group is turned with the decor away from the upper press belt down towards an imagined middle plane with possible underlying overlay paper webs and possible further decor paper webs placed above, bottom colour paper webs and base paper webs. The decor paper web of the second group is then directed with the decor away from the lower press belt up towards an imagined middle plane with possible overlay paper webs placed above and possible further underlaying decor paper webs, bottom colour paper webs and base paper webs. One or more separation webs are placed between the first group and the second group. Further groups of paper webs with intermediate separation webs can be fed between the press belts together with the first and the second group.
The separation web can here preferably consist of
a) a metal such as aluminium steel and copper or
b) a plastic foil for example of polyamide, polyoxymethylene, polycarbonate, polyfluorethene, polyvinylidene fluoride, polyphenyl oxide, polyether sulphone, polyimide, polysiloxane or
c) paper preferably having a surface coating for example consisting of electron cured acrylate, polyphenyl oxide, polyimide, polyvinylidene fluoride, aluminium or the like.
The plastic foils according to c) above can suitably be reinforced with fiber reinforcement to get an increased endurance. Such reinforcement can for instance consist of glass fibers, carbon fibers, aramide fibers or cellulose fibers.
The separation web is suitably provided with a surface structure and two separation webs are placed back to back with the structure side towards the first respectively the second group of paper webs. Alternatively one separation web provided with a surface structure on both sides is used.
The separation web or webs are suitably heated by
a) preheating for example by means of an infra heater or an induction heater before the feeding of the webs between the press belts and/or
b) heating by means of an electric current passing the webs.
The curing process is hereby accelerated. A preheating is especially useful when separation webs with a high heat capacitivity are used. Metals can be mentioned as example of materials with a high heat capacitivity. In those cases where heating by means of passing an electric current is used, metal foils are preferred. The current is here conduced to and from the foil web via collector rolls placed at both ends of the press. Such a separation web can thereby be used as a heating medium at the pressing, whereby the pressing time can be shortened considerably especially at thicker laminates or at the production of many laminates at the same time in the press.
In addition to bringing about an acceleration of the curing process a preheating of the separation web or webs can be used for decreasing the risk of quality problems caused by differences in coefficient of temperature expansion between the separation webs and the other webs in the press. Such differences can otherwise cause local differences of the thickness of the goods.
Alternatively the laminates can be provided with a surface structure by means of one of more rolls provided with a structure pattern and counter rolls. The still warm laminates fed out of the continuous press are here allowed to pass between the rolls and the counter rolls with the decor side facing the rolls at a continuous or discontinuous pressure between the rolls and the counter rolls. The structure webs and the surface structure of the separation webs are then left out.
Thus, when the laminate formed has passed the press belts, rolls provided with a surface structure are pressed against the decor side of the laminate when the laminate passes between these rolls and their counter rolls. The laminate is then only partially cured. The laminate can then be cut into laminate sheets or rolled. The curing process will continue due to the inherent heat of the laminate. Therefore, the laminate will be fully cured.
The structure rolls and/or the counter rolls can if needed by heated, whereby the curing can be controlled. The counter rolls suitably consisting of metal rolls can also be coated on their press surface with a softer material such as an exchangable paper layer having a thickness of one or a few millimetres. The coating material can also consist of a rubber layer having a thickness of a few millimetres and a hardness of 30 Shore Axe2x88x9250 Shore D depending on the density and the depth of the surface structure. In this way the surface structuring is facilitated especially at deeper surface structures. The laminates can also when they have been furnished with a surface structure be heated for instance with an infra heater or hot air to accelerate the remaining curing.
The structure foil webs are preferably separated from their respective groups of paper webs by rolling them up on rolls when the laminates have passed out from the continuous press. The separation web or webs are removed in a similar way from the laminates. Then the backsides of the laminates are possibly ground. Thereby, the structure foil webs and the separation webs can be re-used. In those cases when a separation web or webs having a surface structure are used also these are preferably removed from their respective groups of paper webs by rolling them up on rolls when the laminates have passed out from the continuous press. Then the respective back sides of the laminates are possibly ground. The grinding of the back side is suitably made continuously on the continuous laminate web produced, but it can also be carried out when said laminate web has been cut to sheets.
The different paper webs which in various combinations are used for the production of the laminate according to the claimed process, suitably comprise
a) decor paper webs consisting of dry paper webs of xcex1-cellulose or a traditional white paper impregnated with a thermosetting resin, the webs being provided with a decorative printing or surface layer and the thermosetting resin in the decor paper webs preferably consisting of melamine-formaldehyde resin
b) overlay paper webs consisting of dry paper webs of xcex1-cellulose impregnated with a thermosetting resin consisting of melamine-formaldehyde resin
c) bottom colour paper webs preferably consisting of dry paper webs of xcex1-cellulose or a traditional white paper impregnated with a thermosetting resin preferably consisting of melamine-formaldehyde resin
d) base paper webs consisting of conventional dry underlayer paper webs impregnated with a thermosetting resin, the thermosetting resin in the web placed closest to the decor paper web possibly consisting of melamine-formaldehyde resin while the rest of base paper webs preferably contain phenol-formaldehyde resin or mixtures of melamine-formaldehyde resin and phenol-formaldehyde resin. The various paper webs are dried after the impregnation. The solvent of the resin is evaporated at said drying and the thermosetting resin is partially cured to so-called B-stage. Then the dry paper webs are continuously laminated together at an increased pressure and an increased temperature.
The pressure in the press is 5-80 bar, preferably 20-70 bar, most preferably 30-70 bar especially 30-55 bar and the temperature if 80-250xc2x0 C., preferably 150-200xc2x0 C.
At least one of the paper webs impregnated with a thermosetting resin, preferably the outermost web is coated with hard particles for example silica, aluminium oxide and/or silicon carbide having an average particle size of less than 1000 xcexcm.